13 low amounts when present in marine plants or on the land. None of the radioactive transition elements or only low levels are present in island soil, in plants growing in the soil, or in the herbivorous field rats. However, in the plankton, invertebrate filter feeders and omnivores, the marine the and in the fishes, transition elements may contribute up to 100 per cent of the total radioactivity. Other observations on the biological uptake of radioisotopes are as follows: Of the long-lived fission product isotopes--Sr°”, csh3? and 144 . Ce 4 --only ce l44 enters into the biological cycle in the marine : environment to a : tee significant extent. However, on land, Sr” 0 and cs)3? are taken up but ce l44 is not, to any great extent. Of the gamma-emitting isotopes present in ten tissues of reef and lagoon fish the radioisotopic composition did not vary greatly from tissue to tissue except that radiocobalt was not found in the bone. in fish. Practically no fission products were found The transition elements ranked in order of abundance are radioiron, zinc, cobalt and manganese. Oceanic fish differ from reef and lagoon fish in that radiozinc ranks first in abundance in the former. In comparison with fish, plankton have greater amounts of radiocobalt and lesser amounts of radioiron.